(1) Field of the Invention
The present invention relates to transparent heater panels and a process for preparing the same. More particularly, it relates to transparent heater panels which can be used for window portions, especially transparent heater panels which can be used for liquid crystal displays, refrigerator showcases, freezer showcases, defrosters of cars, and a process for preparing the same.
(2) Description of the Related Art
In freezer showcases and refrigerator showcases, it is necessary to prevent dew condensation on the surface of a windowpane in the window portion. Therefore, a transparent conductive layer was formed on the surface of a windowpane to apply predetermined electric power to the transparent conductive layer and heat the windowpane's surface.
The demand for liquid crystal displays has increased these days. Their operation is, however, inconveniently slow when they are used in cold areas. Thus there has also been an increased need for transparent heater panels with liquid crystal displays for temperature control. Heretofore, in liquid crystal displays used in cold areas, exothermic resistors for heating were used, as disclosed in, for example, JP-A-58-126517 (Tokkaisho). It was, however, difficult to uniformly heat the whole liquid display using one of the constitutions, and the exothermic resistors comprising an opaque metal tended to inconveniently disturb an operator in watching the display.
Transparent heating elements in which transparent conductive layers are formed on transparent substrates are disclosed in, for example, U.S. Pat. No. 4,952,783. One of such heating elements is in FIG. 31 attached hereto. A transparent conductive layer 52 is formed on the whole surface of a transparent substrate 51, and a pair of electrodes 53 and 53' for applying an electric power to the transparent conductive layer 52 are formed over both edge portions of the transparent conductive layer 52. Furthermore, a transparent protective layer 54 is formed on the whole surface of the heating element to protect the transparent conductive layer 52 and the electrodes 53 and 53'. The electrodes 53 and 53' are formed by coating suitable portions of the transparent conductive layer 52 with a conductive printing material such as a silver paste using a screen printing method, and then thermally treating the resulting coated material. For the purpose of improving the reliability of the electrodes, JP-A-4-289685 (Tokkaihei) discloses electrodes sandwiching a metal foil with electrically conductive printing material layers.
The present inventors found that a transparent heater panel is obtained by forming a substantially light-transmittable metallic thin layer on a transparent conductive layer and then forming a pair of metallic electrodes on the metallic thin layer using a wet plating process, as disclosed in JP-A-6-283260 (Tokkaihei). Typical examples of transparent conductive layers used for transparent heater panels are laminates in which a metallic thin layer is held between transparent thin layers of high refractive index. This application also discloses laminates having a sandwiched structure such as InOx/Ag/InOx, SiNx/Ag/SiNx or TiO.sub.2 /Ag/TiO.sub.2, which is formed by a vacuum deposition method, reactive deposition method or sputtering deposition method. When a metallic thin layer containing silver as a main component is used for a metallic layer of a laminate, the resulting laminate has particularly excellent transparency in the visible light range, better conductivity, especially excellent heating performance at a low electric voltage.
In a transparent heater panel whose laminate is covered by a transparent thin layer of high refractive index, and which comprises a thin layer containing silver or copper as a main component and is used as an exothermic body, there is a demand for severe environmental durability when the heater panel is used for the temperature compensation of a liquid crystal display in a car and the like. In particular, there are demands for moisture-heat resistance, heat resistance and cold resistance. In estimation of its moisture-heat resistance, in particular, fine silver particles are generated and coagulated, and they cause the thin layer of silver to deteriorate. Due to such deterioration, the heater panel is dotted with white spots resulting from the coagulation of fine silver particles, its appearance is deteriorated and the generation of heat is not uniform.
Therefore, conventional transparent heater panels are protected by their substrates and protective resins for the purpose of preventing their thin layers of silver from deteriorating. However, when transparent heater panels protected like that are tested for environmental durability, their thin layers of silver are deteriorated. That is, they do not have sufficient environmental durability. In a transparent heater panel in which a thin layer of a semiconductor such as indium oxide is used as an exothermic layer, even if both surfaces are protected, the deterioration of the heater panel is hastened in the presence of acid components, and so on. That is, such a heater panel does not have sufficient environmental durability.
As mentioned above, in transparent heater panels with both surfaces protected, the study of the cause of such deterioration and the solution thereof have been extremely important problems.